Method of removing contaminants from steel melts

ABSTRACT

Described herein is a method of removing contaminants such as oxygen and sulfur from steel which includes the step of adding to a molten steel melt a mixture of a reactive alkali or alkaline earth metal and a halide of the metal adjusted so that the proportion of metal does not exceed its solubility in the halide.

United States Patent 1 1 3,615,354

[72] Inventor Sundaresan Ramachandran 2,036,576 4/1936 Hardy 75/58 X Natrona Heights, Pa. 2,705,196 3/1955 Wever.... 75/58 [21] App]. No. 725,510 3,212,881 /1965 Dunn 75/58 X [22] Filed Apr. 30,1968 3,295,960 l/1967 Parlee 75/58 X Patented Oct. 26, 1971 3,309,194 3/1967 Dunn 75/58 X [73] Assignee Allegheny Ludlum Steel Corporation 3.314.732 9 maud 5/5 Brackenridge, Pa. 3,415,642 12/1968 Matsumoto 75/58 X 2,805,932 9/1957 Menzen... 75/58 2,819,956 l/l958 Strauss 75/57 [54] METHOD OF REMOVING CONTAMINANTS FROM 2,873,188 2/1959 Bieniosek 75/l 30 STEEL MELTS 1 3,065,070 11/1962 Otani 75/57 7 Claims, No Drawings Primary Exammer--L. Dewayne Rutledge [52] US. Cl 75/58, Assistant Examiner-Ii. K. white 75/129 Attorneys-Richard A. Speer, Vincent G. Gioia, Howard R. [51] Int. Cl C21c 7/02, Berkenstock, Jr. and Robert F. Dropkin C21c 7/06 Field ofSearch /58, 130,

ABSTRACT: Described herein is a method of removing con- [56] References cued taminants such as oxygen and sulfur from steel which includes UNITED STATES PATENTS the step of adding to a molten steel melt a mixture of a reac- 771,645 10/1904 Kugelgen 1,335,370 3/1920 Ellis 1,922,037 8/1933 Hardy tive alkali or alkaline earth metal and a halide of the metal adjusted so that the proportion of metal does not exceed its solubility in the halide.

METHOD OF REMOVING CONTAMINANTS FROM 7 STEEL MELTS This invention relates to an improved process for removing contaminants such as oxygen from steel.

In the manufacture of steel, one of the aims is to obtain molten metal that is substantially free of contaminating ingredients. Among the more residual materials more difficult to remove that may adversely affect the mechanical and physical properties of the resultant cast metal is oxygen although other impurities such as sulfur may also be present. It is very desirable to reduce these impurities to a level which is acceptable for the grade of steel being manufactured. In nonfree-machining grades of steel, for example, it is conventional practice to maintain the maximum sulfur content as about 0.03% and it is considered highly desirable to limit the oxygen content of all stainless steel grades at less than about 0.01%.

In conventional steel melting, impurities such as oxygen and sulfur are removed from a molten bath by adding materials to the furnace which form a slag that floats to the surface of the molten metal. Such a slag typically contains lime (calcium oxide) which serves to remove the sulfur by reacting with iron sulfide or other metallic sulfides to form calcium sulfide. Oxygen is generally removed from the bath by the oxidation of various metallic materials such as manganese, aluminum, or silicon, or by combining with carbon to be removed as carbon monoxide gas. Such conventional methods take considerable time and frequently result in entrained inclusions as a result of small amounts of calcium oxide, calcium sulfide, silicon oxides and other similar compounds being trapped within the molten metal. If such compounds do not rise to the surface to be removed with the slag, they will remain in the metal and'solidi fy upon teeming to form undesirable nonmetallic inclusions.

It has also been known to add halides of calcium, such as calcium chloride and calcium fluoride to the molten steel as a flux to accelerate the removal of the inclusion-forming materials, e.g., calcium oxide, calcium sulfide and silicon oxide. However, the halides do not react with oxygen or sulfur within the melt and, therefore, do not contribute to their removal.

Alkali and alkaline earth metals in elemental form are highly reactive and would effectively combine with both sulfur and oxygen if they could be supplied to the molten steel in sufficiently pure form. However, these metals in elemental form are so'violently reactive in molten steel that they volatize or oxidize explosively before they could possibly be brought into intimate contact with the steel to react with the contaminants.

The present invention provides a method whereby alkali and alkaline earth metals, i. e., metals in Groups I and II of the Periodic Table, may be supplied to molten steel in such a manner that it will not oxidize or volatilize prior to entering the molten steel bath. By practicing the invention, such elements may be used to combine with contaminants, e.g., oxygen and sulfur, in the bath to effectively reduce the quantities of such impurities and put them in a form that may be removed by fiuxing. It has been found that alkali and alkaline earth metals may be added to molten steel as a mixture of the metal and its halide salt without the difiiculties previously described. The proportions of the reactive metal and halide in the mixture must be carefully controlled, however, to accomplish reaction with contaminants without danger of explosion. This can be achieved if the proportions of reactive metal and halide salt are such that the concentration of metal does not exceed its solubility in the respective halide and the reaction products, e.g., deoxidation products, do not exceed their solubility in the halide flux. If one or both of these conditions are not met then either the reaction will be violent or the deoxidation products will not be completely fluxed from the bath.

Some prior attempts have been made to purge oxygen and sulfur from molten steel which have purportedly included the addition of calcium monofiuoride. This approach is disclosed in U.S. Pat. No. 2,915,381. However, these efforts have been unsuccessful because in the materials used, the calcium was present in such an amount (about 60% elemental calcium) which exceeded its solubility in the halide thereby causing a violent reaction which destroys the usefulness of the calcium fluoride and results in teemed metal having excessive amounts of oxide and sulfide inclusions. However, it has now been discovered that if mixture of alkaline earth alkali metal and its respective halide is used in proportions such that the quantity of metal in the mixture is maintained at a-level below that of'its actual solubility in its corresponding halide salt, it is possible to add the mixture to molten steel withoutexperiencing; a violent reaction or explosion. In this way, the reactive metal is free to combine with sulfur and oxygcnpresent in the steel to form calcium oxide and/or calcium sulfide but at a safe rate. The halide salt remains within the bath and acts asafiux to facilitate removal, via the slag, of metal oxides and metal sulfides formed by the reactive metal addition. The result is that the oxygen and sulfur level of the steel is materially reduced and a substantially inclusion-free steel may be obtained.

Since calcium is the least expensive and most plentiful of the useful alkali and alkaline earth metal currently available, the following examples describing the practice of the invention disclose this metal and its halides. for the purposeof deoxidizing and desulfurizing molten steel. However, it is understood that other alkali and alkaline earth metals may be employed equally as well and the principles described in connection with the calcium in the ensuing examples are also applicable to these other metals.

EXAMPLE I- A l00-pound induction furnace heat of Type 302 was melted and treated in the ingot mold with a molten mixture of calcium dissolved in calcium chloride. The molten calciumcalcium chloride mixture was made by premelting 2770 grams CaCl, in an iron crucible under an inert atmosphere and adding36 grams of calcium metal chips to the molten CaCl,. Themelt was stirred with an iron rod to aid the dissolution of the calcium metal. Just prior to the heat being tapped, the molten Ca-CaCl mixture was poured into the mold. The heat was immediately tapped on top of the molten mixture. A sample taken from the teemed stream showed an oxygen content of 0.028%. The average oxygen content of the ingot was 0.0 l7%. This is a decrease in oxygen of 39%.

Example ll A l20-pound induction furnace heat of Type 302 was melted. Half of this heat (60 pounds)=was tapped into amold containing a molten mixture of 14 grams calcium dissolved in grams calcium chloride. This mixture was made under the conditions of Example I. The balance of the heat (60 pounds) was treated in the furnace by adding a molten mixture of l4 grams calcium dissolved in 80 grams CaCl (made under the conditions of Example l) into the furnace and stirring the bath with a rabble rod. Thereupon, the heat was tapped. Analysis showed that an untreated sample from this heat contained an oxygen level of 0.028% oxygen. The average oxygen in the ingot treated in the mold was 0.0096, which corresponds to a 65% decrease in oxygen. The average oxygen in the furnace treated ingot was 0.0047 which corresponds to an 83% decrease from the untreated sample.

EXAMPLE III A mixture of calcium dissolved in calcium-chloride was prepared by melting 453 grams of CaCl in an iron crucible under an inert atmosphere at 820 C. and adding 59 grams of calcium in the form of metal chips to the crucible when the chloride was molten. The mixture was stirred with an iron rod to aid the dissolution of calcium in the chloride when the calcium metal dissolved. This material was poured onto a chill plate, cooled, crushed and charged into a moisture free Airco Batch fluidizer. This mixture was then blown into the teemed stream of a l20-pound induction heat of Type 302. Analysis showed that an untreated sample has an oxygen content of 0.029% oxygen. The average oxygen content from the ingot was 0.0 l 35% oxygen, which is at 54% decrease.

EXAMPLE IV A 120-pound heat of Type A-286 was melted in an induction furnace. One-half of this heat (60 pounds) was cast as a standard ingot. The other half of this heat was treated in the furnace with a molten mixture of 40 grams of calcium dissolved in 230 grams of CaCl prepared in accordance with the procedure of Example 1. The average oxygen content of the standard ingot was 0.0063% oxygen, and the average oxygen content of the treated ingot was 0.005l% oxygen. This corresponds to a 19% decrease in oxygen content.

EXAMPLE V A 23-ton arc furnace heat of Type 304 was melted and tapped into a 25-ton ladle. This ladle teemed 27 ingots. The 24th ingot of this teem was treated in the stream with a molten mixture of 1.3 pounds calcium, 1.07 pounds Caland 8.6 pounds CaCI This mixture was prepared as follows. The CaCl and Cal} were melted together in an iron crucible under an argon atmosphere. When this mixture was molten, the calcium in the form of chips was added to the chlorides, and argon was blown into the mixture to aid the dissolution of calcium in the molten halide salts. When the entire mixture was molten, the material was poured into a launder so as to combine with the first portion of steel being teemed into the tundish of a 12 inches X 12 inches square ingot mold. The average oxygen content of a standard ingot from this heat was 0.0069% oxygen. The average oxygen content of the treated ingot was 0.0043% oxygen. This corresponds to a 38.8% decrease in oxygen.

As discussed above, there is 2 finite compositional limit for the alkali or alkaline earth metals useful in accordance with the invention. This limit is based upon the solubility of the metal in its halide or upon the solubility of the reaction produce in the halide salt whichever is lower. Maximum solubility of some of the metals and metal oxides of the elements useful in accordance with the invention in their respective halide salts is disclosed in Table 1.

TABLE I Solubility and Vapor Pressure Duta on elected Megll Metald-lalide Systems Max. Solubility Vapor Pressure Theoretically there is no lower solubility limit of the metal in its halide, and mixtures having very little reactive metal would have a beneficial result which corresponds to the amount of reactive metal used. It has been observed, for example, that concentrations of as little as 1% of the metal have been dissolved satisfactorily in its halide to form a mixture which has been useful in deoxidizing molten steel. However, for practical purposes a lower limit of a 1%-solution of the metal in its halide is generally regarded as a preferred minimum.

It is apparent from the foregoing examples that best results are obtained by forming the metal-halide mixture by heating the halide salt in a furnace under an inert atmosphere until liquified. Elemental metal is then added in such an amount as to effect the desired concentration. The molten metal is stirred or agitated until dissolution is complete and after co olmg m a controlled atmosphere to solidify, the metal-halide mixture is crushed to a size which may be conveniently added to the steel furnace or stored until it is ready for use. Advantageously, the mixture should be kept dry during storage.

The amount and specific material used to accomplish removal of contaminants in a give metal heat is dependent entirely upon the purity of the bath and the raw materials used as well as the desired final level of impurities.

1 claim: 1. A method for removing oxygen and sulfur from steel which comprises:

preparing a mixture consisting essentially of (1 metal selected from the group consisting of alkali and alkaline earth metals and (2) at least one halide of the same metal. the proportion of metal to halide in said mixture being such that the amount of metal in said mixture does not exceed its solubility in the halide in said mixture, said metal being present in solution in said halide in said mixture;

adding said 'mixture to molten steel whereby the metal in said mixture reacts with oxygen and sulfur present in said steel and the halide in said mixture fluxes the oxides and sulfides formed thereby; and

removing said oxides and sulfides from said steel, together with said flux.

2. A method according to claim 1 wherein said reactive metal and halide mixture contains at least 1% metal.

3. A method according to claim 1 wherein said reactive metal is calcium.

4. A method according to claim 3 wherein said halide is calcium chloride.

5. A method according to claim 1 wherein the oxygen content of said steel melt is lowered to less than about 0.01%

6. A method according to claim 1 wherein said other contaminants include sulfur and wherein the sulfur content of said steel melt is lowered to a maximum ofabout 0.03%.

7. A method according to claim 1 wherein said mixture of reactive metal and halide is added in solid form to melt and then mixed into said steel melt. 

2. A method according to claim 1 wherein said reactive metal and halide mixture contains at least 1% metal.
 3. A method according to claim 1 wherein said reactive metal is calcium.
 4. A method according to claim 3 wherein said halide is calcium chloride.
 5. A method according to claim 1 wherein the oxygen content of said steel melt is lowered to less than about 0.01%
 6. A method according to claim 1 wherein said other contaminants include sulfur and wherein the sulfur content of said steel melt is lowered to a maximum of about 0.03%.
 7. A method according to claim 1 wherein said mixture of reactive metal and halide is added in solid form to melt and then mixed into said steel melt. 